The invention relates to a reactor for a two-phase or three-phase system.
The invention has particular application to the aeration of a fluid comprising a slurry of mineral particles with air or any other suitable oxygen-containing gas, as is required by way of example in aerobic bacterial leaching. However, the invention is not restricted to this application and extends to the aeration of any gas/liquid, gas/liquid/solid, or gas/liquid/solid/microbial systems.
The invention has the advantages of aerating a fluid with a gas at low energy usage and with high efficiency in terms of gas utilisation.
The term xe2x80x9caerationxe2x80x9d is understood to mean herein the introduction of a gas or gases into a fluid.
Reactors for aeration of slurries have been in use for many years in the mining industry. The two major types of reactors are the Pachuca (or air agitated reactor) and the mechanically agitated reactor.
The Pachuca reactor was initially favoured due to its simplicity of construction and operation but gradually lost favour as reactor size increased. The loss of favour resulted from the large amounts of compressed air required for good mineral suspension. Also, the residence time of air in a Pachuca reactor is too short for efficient mass transfer and Pachuca reactors are prone to channelling of the air. Air agitation, in general, is inefficient because the bubble size for efficient agitation is too large for efficient mass transfer.
Mechanical agitation has become more widely used, particularly for large reactors, as impeller design has become more efficient and it has become evident that the extra capital cost was more than compensated for by the relatively lower energy required for agitation.
For efficient mass transfer of air to solution it is necessary to obtain a fine dispersion of bubbles in a well mixed system with the bubbles having a long residence time in the reactor. In practice this has been obtained by passing the air through a high shear turbine impeller or by introducing the air through a membrane or porous diffuser. Both these methods are energy intensive, because the air must be introduced at sufficient over-pressures to overcome the liquid pressure at the point of injection and to overcome the pressure drop across the injection opening, membrane or diffuser. Usually, the point of injection is at the bottom of the reactor and, in particular, in the case of aerating large vessels, one of the major costs is the capital and on-going energy costs to compress the air to the pressure required for injection. If the tanks are deeper than about 10 m it is necessary to install expensive, high pressure, compressors rather than air blowers. Additionally, the use of porous diffusers, or spargers, in reactors for slurries can lead to loss of operating time to unblock the diffusers.
In addition, mechanically agitated reactors become inefficient when large amounts of air are required, because the power required to disperse the air in the reactors becomes very large. Further, in the case of bacterial reactors, the shear forces present at the blade tips of high speed impellers can damage the bacteria.
In addition, particularly for gas/liquid/solid systems where it is important to maintain the solids in suspension, the power required to circulate the fluid in the aerator becomes a significant cost factor.
According to the invention there is provided a reactor for introducing a gas into a fluid comprising, a mixing tank for the fluid, a partition means for dividing the tank into at least two chambers and for allowing the fluid to flow between the chambers at a lower region and an upper region of the tank, a pump means located in one of the chambers for circulating the fluid downwards in one chamber and then upwards in the other chamber, a means for creating a region of reduced pressure in a portion of the fluid, a means for introducing the gas into the fluid in the region of reduced pressure to aerate the fluid, and a means for introducing the aerated fluid into the circulating fluid in the tank.
It is preferred that the partition means comprises a draft tube adapted to be submerged in the fluid in the tank, the draft tube having an open upper end and an open lower end.
It is preferred particularly that the tank be cylindrical and the draft tube be located centrally in the tank to divide the tank into an inner chamber and an outer annular chamber.
It is preferred that the pump means be located in the draft tube.
It is preferred that the pump means comprises an axial flow pump.
It is preferred particularly that the axial flow pump comprises an impeller located in the draft tube.
It is preferred that the means for creating the region of reduced pressure in the fluid comprises a tubular member which has a region of restricted cross-section for imparting a venturi effect to the fluid passing through the tubular body whereby the velocity of the fluid increases and the pressure of the fluid decreases in the region of restricted cross-section.
In one preferred arrangement the region of restricted cross-section is formed by providing a throat in the tubular member. In another preferred arrangement the region of restricted cross-section is formed by inserting a restriction in the tubular member.
It is preferred that the means for introducing the gas into the fluid comprises a porous membrane, holes, or jets.
According to the invention there is also provided a method of introducing a gas into a fluid comprising, circulating the fluid by means of a pump means in a mixing tank having at least two chambers that are in fluid communication at upper and lower regions of the tank so that the fluid flows downwards in one chamber and upwards in the other chamber, creating a region of reduced pressure in a portion of the fluid, introducing the gas into the portion of the fluid in the region of reduced pressure to aerate the fluid, and introducing the aerated fluid into the circulating fluid in the tank.